For example, in shipping and aviation, the standardized communication system AIS is widely used for local communication between, for example, moving vehicles or between moving vehicles and fixed installations, such as a port or a lighthouse. See for example, “Recommendation ITU-R M.1371-4”, published 2010 by the International Telecommunication Union.
Under this system, a set of digitally-coded, predefined messages of different types can be used by for instance a vessel in order to, among other things, inform recipients in local geographical proximity to the craft in question of its present position and speed. The different available message types are predefined in the sense that their outer form must comply with a predefined template, including the frequency band use, transmission interval and so on. Furthermore, each message must, apart from the information-carrying content, contain a certain opening and a closing binary number sequence, respectively, a checksum, and so on. Finally, the form used for the information bearing content of most messages is predefined in such a way that the binary positions in each respective message's information-carrying part comprises information of a given predefined type, such as the vehicle's current speed, on a certain predetermined binary number format.
There are also a number of messages whose information-carrying content (the so-called payload) can be used for transmission of binary coded information of general character, where a given subset of those binary positions which constitute the message can be used to transfer freely selectable such information. However, in practice the use of these messages for such purposes is strongly limited, because such use is subject to centrally issued recommendations. In other words, there is a degree of standardization also regarding which type of information that can be transferred via such general messages.
One reason for this far-reaching standardization of the AIS system is that there is also a standardization regarding the functionality of the different types of transmitters and receivers, which are to be able to put together and interpret, respectively, AIS messages sent between them. This means not only that the outer form and content of AIS messages must be consistent with that which is prescribed by the standard, but also, for example, that checksums are correct.